1. Field of the Invention:
The present invention relates to a developing unit for use in an image forming apparatus such as a printer, a copier and a facsimile to which an electrophotographic process, an electrostatic recording process or the like is applied and to the image forming apparatus using the same or more specifically to a developing unit for developing an electrostatic latent image formed on an electrostatic latent image carrier by applying oscillatory bias voltage to the developer carrier and to an image forming apparatus using the same.
2. Description of the Related Art:
Hitherto a technology of applying oscillatory bias voltage to a developer carrier has been widely used for a developing unit for developing an electrostatic latent image in an image forming apparatus such as a printer, a copier and a facsimile to which an electrophotographic process or an electrostatic recording process is applied.
Such developing unit is constructed so as to create an oscillating electric field between the developer carrier and the electrostatic latent image carrier by applying the oscillatory bias voltage to the developer carrier and to fly the developer reciproactively between the developer carrier and the electrostatic latent image carrier by the oscillatory electric field to improve the developing efficiency and to prevent fogging and the like.
Then, the image forming apparatus to which the developing unit described above is applied is arranged so as to fly the developer reciproactively between the developer carrier and the electrostatic latent image carrier by applying the oscillatory bias voltage in which DC voltage is superimposed to AC voltage to the developer carrier during the image forming operation to improve the developing efficiency and to prevent fogging and to stop the application of the oscillatory bias voltage to the developer carrier when the image forming operation ends
However, the prior art technology described above has has the following problems. That is, in the image forming apparatus to which the conventional developing unit is applied, the developer in the middle of the developing process in the developing area remains on the surface of the electrostatic latent image carrier regardless whether there is any electrostatic latent image or not when the application of the oscillatory bias voltage which has been oscillated for the development is stopped as the image forming operation end. More specifically, when the application of the oscillatory bias voltage is stopped, the developer which has adhered on the surface of the electrostatic latent image carrier remains while adhering on the surface of the electrostatic latent image carrier regardless whether there is any electrostatic latent image or not by Van der Waals force and image force because no force for pulling back the developer to the side of the developer carrier acts on the developer adhering on the surface of the electrostatic latent image carrier.
When the developer remains on the surface of the electrostatic latent image carrier even after the image forming operation, the developer flies within the image forming apparatus, thus bringing about problems of contaminating the inside of the image forming apparatus or of increasing an amount of unnecessary developer as it is recovered by a cleaning unit.
FIG. 16 is a graph showing the relationship between the phase of AC component and the amount of remaining developer when the application of the oscillatory bias voltage is stopped.
As it is apparent from this graph, much toner remains on the surface of the photographic drum depending on the phase of the AC component when the application of the oscillatory bias voltage is stopped, thus contaminating the inside of the image forming apparatus or increasing the unnecessary developer.
Still more, an image forming apparatus using a contact type roller and a brush which contact with the surface of the electrostatic latent image carrier has come to be put into practical use lately as means for transferring developed image data to a transfer medium such as paper. Such image forming apparatus using the contact type roller and the bush has had a problem that developer remaining on the surface of the electrostatic latent image carrier transfers to contact type charging means such as the roller and the brush which contact with the surface of the electrostatic latent image carrier, thus contaminating the contact type charging means such as the roller and the brush.
It has had also another problem that when the surface of the contact type transfer means is contaminated by the developer, the developer transferred to the contact type transfer means adheres on the back of the transfer medium such as paper, thus causing unnecessary contamination and dropping the image quality in printing on the double sides in particular.
Then, there have been technologies for solving the problem caused by the developer remaining on the surface of the electrostatic latent image carrier when the application of the oscillatory bias voltage is stopped as disclosed in Japanese Patent Laid-Open No. Hei. 2-130569 and Patent No. Hei. 4-35074.
The image forming apparatus disclosed in Japanese Patent Laid-Open No. Hei. 2-130569 has been arranged so as to control the oscillatory phase in stopping the oscillatory bias voltage to reduce the developer remaining on the surface of the electrostatic latent image carrier.
The image forming method as disclosed in Japanese Patent No. Hei. 4-35074 has been arranged so as to stop the oscillatory bias voltage by reducing its output amplitude gradually.
However, although the image forming apparatus disclosed in Japanese Patent Laid-Open No. Hei. 2-130569 has been arranged so as to control the oscillatory phase in stopping the oscillatory bias voltage to reduce the developer remaining on the surface of the electrostatic latent image carrier. It has had a problem that it is unable to reduce the remaining developer charged in the reversed polarity and that it is not so effective in one-component developing system because there exist developers which are charged in the polarity reversed from the predetermined polarity.
Further, although the image forming method as disclosed in Japanese Patent No. Hei. 4-35074 has been arranged so as to stop the oscillatory bias voltage by reducing its output amplitude gradually, it has had a problem that it is apt to be costly because it often requires a complicated circuit structure for its high tension power source in order to reduce the output amplitude of the oscillatory bias voltage gradually as described above. It is noted than an oscillatory bias voltage generator using a simple oscillating circuit composed of C-R coupling which has been widely used since the past allows the amplitude of the oscillatory voltage to be reduced gradually in stopping the oscillation from its circuit characteristics if only the output amplitude of the oscillatory bias voltage is to be reduced.
However, the recent image forming apparatus has come to be digitized in order to enhance the image quality, a rectangular wave of arbitrary shape is liable to be used instead of a sine wave for the oscillatory bias voltage applied to the developer carrier and multiple frequencies have come to be used for the frequency of the oscillatory bias voltage. Therefore, the oscillatory bias voltage generator using the simple analog oscillating circuit composed of the C-R coupling of the past also has had a problem that it requires the complex circuit structure for the high tension power source and is costly in order to generate the rectangular wave of arbitrary shape and the oscillatory bias voltage of the multiple frequencies and to reduce the out amplitude of the oscillatory bias voltage gradually.